It is known how to manufacture starch hydrolysates for which the dextrose equivalent value (the reductive power expressed in glucose over dry matter, henceforth DE) is in the range 2 to 98 and which, depending on this value, can contain up to 96% of actual dextrose. These different qualities of starch hydrolysates are obtained by selecting the conditions of the hydrolysis of the starch. The nature of the hydrolysis, that is, whether it is acid or enzymatic, also plays a part.
Starch hydrolysates rich in dextrose, although they have numerous spheres of application, are primarily used as a raw material in the manufacture of crystallised dextrose or as a substrate for the manufacture of fructose by isomerisation. For these two applications, the highest possible conversion is sought, that is, the highest possible dextrose content, with a minimum of impurities.
The starch conversion processes using an acid give starch hydrolysates in which the dextrose content does not exceed 85% to 90%. These processes in fact favour the concurrent reactions of reversion and internal dehydration of the dextrose.
The starch conversion processes using both an acid and an enzyme (usually a glucoamylase) give starch hydrolysates in which the dextrose content is never above 93%. In fact, in such processes, the acid hydrolysis of the starch produces highly branched saccharides which resist the action of the glucoamylase.
The starch hydrolysates obtained by double enzymatic conversion with .alpha.-amylase and with amyloglucosidase (or glucoamylase), usually titrate from 93% to 95% of actual dextrose, and contain in the range from 5% to 7% of residual oligosaccharides and polysaccharides, the majority of which consists of disaccharides (maltose and isomaltose).
These hydrolysates are obtained in the usual way by liquefaction of the starch up to a DE in the range from 12 to 20, followed by saccharification to amyloglucosidase, but in these conditions the actual dextrose content cannot exceed 94% to 95%.
In order to obtain higher actual dextrose contents, several processes have been proposed to either improve the conversion of the starch by limiting the formation of co-products or to improve the efficacy of the separation dextrose/co-products (oligosaccharides and polysaccharides).
Thus a first process consists in carrying out the stages of liquefaction and saccharification at very low dry matter contents (in the order of 5% to 10%). But even at such low dry matter levels, the actual dextrose content does not exceed 95% to 97%. In addition, such a process is not at all economically viable because of the energy required for evaporation of the water.
Another process consists in carrying out the saccharification in the presence of an enzyme which hydrolyses the 1-6 bonds of the starch, but even in this case, the dextrose content is only 96% to 97% maximum.
Yet another process consists in separating, by a method known per se, the dextrose and the oligosaccharides and polysaccharides by passing the hydrolysate over a column of a molecular sieve such as a cationic resin. In such a process, the aqueous starch hydrolysate having previously undergone a pre-treatment such as concentration, filtration and/or bleaching, is adsorbed over the column, and the co-products (the polysaccharides and some of the oligosaccharides) end up in the raffinate excluded from the sieve. The dextrose is then desorbed by elution with water, and the water is then partially or completely eliminated to form a concentrated dextrose solution or crystallised dextrose.
Another process, based on the same principle as the one above, consists in separating the dextrose and the oligosaccharides and polysaccharides by passing the starch hydrolysate over tangential filtration membranes. Such a process is described in documents FR-A-2.762.616 and U.S. Pat. No. 5,869,297.
This last process effectively permits the obtention of a starch hydrolysate with a high actual dextrose content of more than 98%-99%, but the yields obtained are unfortunately too low (in the order of 20% to 25%) to justify such processes from the industrial and economic points of view.